\section*{Acknowledgements}

We would like to thank Weiliang Qian and Xiangyang Liu for helping me develop and test the codes.
We also thank Nicole Kwoh and Joanna Izabela Siebert for their technical discussions and proofreading.

This work is supported by the National High Technology Research and
Development Program (863 Program) of China (Grant No. 2006AA01Z172 and
2008AA01Z106), the National Natural Science Foundation of China (Grant
No. 60533040, 60725208 and 60773089), and Shanghai Pujiang Program
(Grant No. 07pj14049). This work is also partially supported by Hong Kong Polytehnic Univ under the grant
G-U513. 
